I don't know if there is a standard schematic symbol. In general, schematics show the components needed to understand the function and behavior of the circuit and connectors are generally not considered functionally or behaviorally significant. So they are often just mapped to the schematic's I/O ports (if that). In RF, of course, the connectors can be significant, but then they are often modeled as lumped parameter circuits. Whether or not that model is tied to a schematic symbol is up to whoever put the symbol library together.